Actually, the login for webmail is the first portion of the email address that you want to log in with. For example, if your email address is 'email@example.com', you would use 'foo' to log in. This is completely independent of the actual mailbox or user account that the email address leads to.
With traditional email clients, the only supported way to log in right now is using your actual mailbox or user account.
(the disparity here is indeed a bit confusing, and we're working on making it so that people can log in using the webmail-type method either way, though that's not yet supported)
The login you use to access the web panel - known as a Web ID - is completely separate from the above, and should be considered a different account entirely. While it may have the same login/password as your email or FTP accounts (some people like to set it up that way), it is a distinctly different type of account.
- Jeff @ DreamHost
- DH Discussion Forum Admin